It is c I hope I helped out with this question!.
Answer:
The mass number is defined as the total number of protons and neutrons in an atom.
Explanation:
Most elements on group 18 are the Noble Gases. They already have a complete last level with 8 electrones. Actually they can form compounds but only on the lab and they will not even last half a second.
Carbon is an atom so electrons=protons
<span>First shell has 2 electrons and the next 8 so with 6 electrons the first shell fills and 4 are left to go into the 2th shell.</span>
